This invention relates generally to an electrophotographic printing machine, and more particularly concerns a transfer apparatus employed therein which produces a color corrected copy from an original document.
In the process of electrophotographic printing, a photoconductive surface is uniformly charged and exposed to a light image of the original document. Exposure of the photoconductive surface creates an electrostatic latent image corresponding to the original document. Toner particles are then electrostatically attracted to the latent image to render it viewable. Subsequently, the toner powder image is transferred to a sheet of support material and permanently affixed thereto to produce a copy of the original document. The foregoing process is described in detail in U.S. Pat. No. 2,297,691 issued to Carlson in 1942.
Multi-color electrophotographic printing is substantially identical to the heretofore discussed process of black and white printing with the following distinctions. Rather than forming a total light image of the original, the light image is filtered producing a single color light image which is a partial light image of the original document. The foregoing single color light image exposes the photoconductive surface to create a single color electrostatic latent image. The single color electrostatic latent image is developed with toner particles of a color complementary to the single color light image. The single color toner powder image is then transferred from the electrostatic latent image to a sheet of support material. This process is repeated a plurality of cycles with differently colored light images and the respective complementary colored toner particles. Each single color toner powder image is transferred to the sheet of support material in superimposed registration with the prior toner powder image. This creates a composite multi-layered toner powder image on the sheet of support material. Thereafter, this composite multi-layered toner powder image is permanently affixed to the sheet of support material to create a color copy corresponding to the colored original document.
The fidelity of the color is limited by the imperfect nature of the spectral transmittance of the toner particles. Ideal toner particles perfectly absorb over a preselected spectral region and perfectly transmit over the remaining spectral region. For example, ideal cyan will perfectly absorb red light and perfectly transmit blue and green light. Similarly, ideal magenta will perfectly absorb green light and transmit both blue and red light. Finally, ideal yellow will absorb perfectly in the blue region while transmitting both red and green light. However, real materials differ from these ideal colorants by exhibiting unwanted absorption in regions where they should be perfectly transmitting. Typical cyan toner particles absorb not only red but also some green thus, cyan toner particles contain some magenta impurities therein. Similarly, typical magenta toner particles absorb some blue and therefore contain some yellow impurities therein. It should be noted that the yellow toner particles are substantially pure. It is therefore apparent that a combination of the foregoing toner particles will produce not only the desired resultant color but a color produced from the impurities which is an undesired effect. This will result in the colors of the copy differing from that of the original. Compensation for the impure spectral characteristics of the toner particles is termed color correction. A set of ideal toner particles would require no color correction.
Thus, it is a primary object of the present invention to improve the method and apparatus employed in reproducing color copies so as to correct for the impurities of colors employed therein.